1. Field of the Invention
The present invention generally relates to wireline coring of subterranean formations, and more specifically to a bit-stabilized combination coring and drilling system offering interchangeable placement and retrieval of coring inner tube assemblies and drilling plug assemblies for drilling ahead, the latter also being optionally provided with logging capabilities.
2. State of the Art
Wireline coring has been known for many years. The basic concept of wireline coring involves the use of a core barrel including an outer barrel assembly disposed at the end of a drill string and having a core bit or crown at the bottom thereof. An inner tube assembly for receiving a core cut by the core bit is releasably latched into the outer barrel assembly. This arrangement permits placement of the inner tube assembly in the outer barrel assembly by wireline, gravity, or hydraulic flow, and retrieval thereof from the outer barrel assembly via wireline. Examples of such prior art wireline coring systems are disclosed in U.S. Pat. Nos. 3,127,943 and 5,020,612, incorporated herein for all purposes by this reference.
One problem with many such prior art systems is the necessity of using a special drill string having an enlarged diameter to accommodate running and retrieval of an inner tube assembly used to cut relatively large cores in excess of two inches in diameter.
While coring systems cutting small or "slim-hole" cores of 1 3/4" or less in diameter are known, it will be appreciated that such cores are extremely fragile and conventional coring systems are limited in the length that such cores can be reasonably cut without fracturing. This limitation appears to be primarily due to instability of the entire core barrel initiated by lateral and vertical bit movement in the borehole, which produces vibration. A major phenomenon resulting from such bit movement and vibration is so-called bit "whirl", although vibration without whiff is still detrimental. The phenomenon of bit "whiff" is exhibited in bits have unbalanced cutter side forces, which forces cause the bit to rotate or "whirl" in the borehole about a center point offset from the geometric center of the bit in such a manner that the bit tends to whirl backwards about the borehole. The whirl phenomenon has been observed to be aggravated by the presence of gage cutters or trimmers at certain locations on the outer gage of the bit, such cutters also generating frictional forces during drilling. Whirl is a dynamic and self-sustaining phenomenon, and in many instances is highly destructive to the drill bit cutters. The whirl phenomenon also causes spiraling of the borehole during drilling which results, in core bits, in a non-cylindrical, spiraled core which is more susceptible to fracture, and jamming in the core barrel inner tube.
Given the relatively small clearances between the core and the pilot shoe, core catcher and inner tube components of the inner barrel, slight lateral and vertical movements of the core barrel easily result in fracture of small-diameter cores with attendant core jamming and degradation of the core sample. As a result, small diameter core barrels have been traditionally limited in length due to the short (for example, ten to thirteen foot) core Samples which could be cut without experiencing the aforementioned core fracture, jamming and degradation. Attempts have been made to cut longer cores, as long as twenty-six feet, but the apparatus employed has never been deemed successful due, again, to the aforementioned problems.
It has been recognized that certain recent improvements in bit design, including but not limited to the so-called "anti-whirl" polycrystalline diamond compact (PDC) cutter bits initiated by Amoco and improved by the assignee of the present invention, could be applied to core bits to enhance the reliability of a coring operation and the quality of the cores. Patents disclosing anti-whirl bits include, without limitation, U.S. Pat. Nos. 4,982,802; 5,010,789; 5,042,596; 5,099,934; 5,109,935; 5,111,892; 5,119,892; 5,131,478; 5,165,494; and 5,178,222, the disclosures of which are incorporated herein by this reference. SPE (society of Petroleum Engineers) Paper No. 24587 by L. A. Sinor et al of Amoco Production Co., entitled "Development of an Anti-Whirl Core Bit", discusses improvements and potential improvements in coring capability thought to be offered through the use of anti-whiff core bits.
Other approaches to bit stabilization have been taken, by Amoco as well as others. One approach is to attempt to perfectly balance a bit, as disclosed in U.S. Pat. No. 4,815,342, the disclosure of which is incorporated herein by reference. Another approach is to mechanically "lock" the projections on the bit face into circular grooves cut by the cutters on the face, as disclosed in U.S. Pat. No. 5,090,492, the disclosure of which is incorporated herein by reference.
All of the foregoing developments in bit stabilization have been focused on discrete elements of the drilling operation, either drilling a full-gage wellbore or in coring.
Some years ago, Eastman Christensen Company, a predecessor to the assignee of the present invention, developed a combination drilling and coring system having a "Drill-Core System" option, which allowed for alternate coring and drilling operations without tripping the drill string. In the Drill-Core System, both the inner barrel assembly for coring and a substitute center plug assembly with a crowsfoot and cutters for converting the core bit to a drill bit were deployable and retrievable via wireline. The Drill-Core System employed natural diamond core bits, and was only marginally successful for several masons. First, the maximum core length which could be cut at one time was only thirteen feet, providing an extremely short interval for analysis without multiple trips of the inner tube assembly, and requiting combination with odd-length tubulars to drill the kelly down to the rotary table like a pipe joint. In addition, the advent of more accurate electric well logs and analysis techniques for logging data reduced the demand for core analysis. Finally, the industry was not accepting of the relatively small diameter cores (2") taken by the system, which was required in order to deploy and retrieve the inner barrel assembly and center plug assembly through standard tubular goods.
In recent years, however, the development and industry acceptance of punch-and rotary-type sidewall coring techniques which result in 1" diameter cores from the side of the borehole being drilled, as well as the increased use of slim-hole drilling for exploratory wells has eliminated the prior hesitancy to accept and rely upon small-diameter cores. These changes in industry practices have resulted in a renewed interest in coring, but to date state of the art coring systems have not offered an acceptable slim-hole coring and drilling system, which can cut pristine, undamaged cores of a desirable length (for example, thirty feet), substantially avoid core jamming, and also provide a capability for drilling ahead between intervals to be cored without tripping the drill string. Moreover, no state of the art coring system offers performance capabilities and operating characteristics similar to those of PDC drill bits.